The United States hasn't won an Olympic medal in cross-country skiing since 1976, but in 2010, several skiers hope to change that. If they're successful, you can be certain it's due to their incredible endurance--cross-country skiers are among the fittest athletes in the world. Deborah King, an associate professor in the Department of Exercise and Sports Sciences at Ithaca College and Joseph Francisco, president of the American Chemical Society, look at the biochemistry of human endurance.